Color is one of the most important aspects of printing inks and paints. Printing inks are used for generating images and text on a variety of consumer and industrial goods. A primary function of the image and text is to identify and differentiate the printed object. The value of image and text can be directly related to the quality of the image including its color. One of the primary purposes of paints is also to provide a specific color to the underlying substrate. The value of a paint can also be directly related to its ability to provide a specific color.
Nearly all printing inks and paints contain three basic ingredients, a colorant, a binder and a carrier fluid. The colorant may be a pigment or a dye. The binder is used to fix colorant on a substrate and to impart end use properties such as water and chemical resistance. The carrier fluid may be water, an organic solvent or a mixture of water and organic solvent. Printing inks and paints often contain other additives to impart specific properties. For example, surfactants may be used to improve wetting of the substrate, bases such as amines may be used to control the pH of the ink and extender pigments may be used to impart bulk to a paint.
The color of a printing ink and paint is primarily derived from the pigments or dyes used as colorants. Color strength is a measure of the ability to impart color. In the case of inks and paints, color strength is mostly a function of the amount of pigment or dye contained in the ink or paint. There is a need to formulate inks and paints which have adequate color strength with the minimum amount of pigment or dye. One reason for reducing the amount of pigment or dye is that these ingredients are costly. Other reasons for reducing the amount of pigment or dye contained in inks and coatings are due to environmental hazards associated with these ingredients. Recycling of printed materials such as newspaper, magazines, brochures, packaging materials, and the like is done to reduce the amount of waste sent to land fills. During the process of recycling, printed materials are subject to various processes including removal of inks. Removal of colored species such as pigments and dyes is necessary as the value of recycled material is also related to its color. For example, recycled paper with no color can find greater use and therefore can have higher value than colored recycle paper. A reduction in the amount of pigments or dyes present in printing inks would be beneficial to obtaining recycled materials with no color.
Another reason for reducing the amount of colorant in inks is related to the need to use recycled paper and paperboard in a number of applications. The use of recycled paper is made in order to conserve natural resources and to reduce cost. However, additional processing is required in order to achieve a print quality on recycled paper which is comparable to that on fresh stock. Some of the additional processing is needed in order to bleach the recycled stock. White paper is desired for better print quality. However, a color composition which is able to hide the color of recycled paper would eliminate the need for bleaching and other processing of recycled paper.
One approach to producing colored inks and coatings without the use of coloring agents, such as pigments, has been to include sub-micron particles, and specifically microspheres, as scatters in the ink or coating. It is known in the theory of light scattering that size of the scattering sites has an influence on the wavelength of the light that is scattered. This phenomenon is seen when the size of the scattering site approaches the wavelength of the incident light. Under such circumstances smaller scattering sites preferentially scatter shorter wavelengths and larger scattering sites preferentially scatter longer wavelengths.
Hollow microspheres have been made in order to control the hue of white or non-pigmented ink-jet inks. For example, U.S. Pat. No. 4,880,465 (Loria et al) teaches a non-pigmented ink suitable for use in ink jet printing. The ink comprises a resin component, hollow microspheres, and a suitable carrier vehicle. The hollow microspheres each contain a central void region filled with a liquid capable of diffusing through the walls of said microsphere and have an inside diameter from about 0.1 to about 0.5 micron and an outside diameter from about 0.4 to about 1 micron. Loria et al further teach that upon drying the ink formulated with these microspheres forms a coating laden with microvoids which effectively scatter light to produce an opaque image. It is also taught that the microspheres which do not contain the microvoid are not suitable.
European Patent No. EP 1544804 (Finley et al) teach a non-pigmented ink composition suitable for use in ink jet printing comprising a carrier liquid and a multi-modal blend of polymer particles, wherein each mode has a particle size of between 0.2 to 1.5 micron, at least one mode is a hollow microsphere polymer and at least two modes differ in particle size by at least 0.1 micron. Finely et al teach that the blending of two or more hollow microsphere components having very different cavity sizes or shapes makes it possible to accurately select any desired hue characteristic of a white ink, from a bluish hue to a pure white.